Implantable wireless sensors are useful in assisting diagnosis and treatment of many diseases. Examples of wireless sensor readers are disclosed in U.S. patent application Ser. No. 12/737,306 entitled Wireless Sensor Reader, which is incorporated by reference herein. Delivery systems for wireless sensors are disclosed in PCT Patent Application No. PCT/US2011/45583 entitled Pressure Sensor, Centering Anchor, Delivery System and Method, which is also incorporated herein by reference. In particular, there are many applications where measuring pressure from within a blood vessel deep in a patient's body is clinically important. For example, measuring the pressure in the heart's pulmonary artery is helpful in optimizing treatment of congestive heart failure. In this type of application, a sensor may need to be implanted 10 to 20 cm beneath the surface of the skin.
Wireless sensors that use radiofrequency (RF) energy for communication and/or power have been found to be particularly useful in medical applications. However, a key challenge in successful commercialization of these implantable wireless sensors is the design tradeoff between implant size and the “link distance”, which is the physical distance between the implant and the external device communicating with the implant. From a medical standpoint, it is desirable for an implant to be as small as possible to allow catheter based delivery from a small incision, implantation at a desired location, and a low risk of thrombosis following implant. However, from a wireless communication standpoint, the smaller the implant, the shorter the link distance. This distance limitation is driven primarily by the size of the antenna that can be realized for a given overall implant size. A larger antenna is better able to absorb RF energy and transmit RF energy than a smaller antenna. For example, in the case of wireless communication via inductive coupling, a typical implant antenna has the form of a coil of wire. The coil's “axis” is the line that extends normal to the plane of the windings, i.e. the axis is perpendicular to the wire's length. As the area encircled by the coil increases, the amount of magnetic flux that passes through it generally increases and more RF energy is delivered to/received from the implant. This increase in flux through the implant antenna can result in an increase in link distance. Thus to achieve maximum link distance for a given implant size, the implant antenna should be of maximal size.
While antenna size is important, other implant architectures may benefit from maximizing the size of other internal components. An implant containing an energy storage device such as a battery, for example, would enjoy longer battery lifetime with a larger battery. In another example, a drug-eluting implant could hold a larger quantity of the drug. Other examples will be apparent to those skilled in the art.
Another challenge in commercialization of implantable wireless sensors is the need to protect the sensitive sensor electronics from potentially corrosive or damaging fluids of the body. For many implant applications, the sensor may need to record accurate measurements for a period of time exceeding 7 to 10 years. Small changes in electrical, chemical, or mechanical properties of the implant over this time period can result in inaccurate measurements. To prevent inaccurate measurements, a hermetic enclosure may be required to protect the sensitive electronics of the sensor from the transfer of liquids and gases from the bodily environment.
Hermetic enclosures for implants are typically constructed of metals, glasses, or other ceramics. Metals are malleable and machineable, capable of being constructed into thin walled hermetic enclosures such as the titanium enclosures of pacemakers. Unfortunately, the use of metals in hermetic enclosures may negatively impact the ability of the sensor to communicate wirelessly with an external device, especially when communication at low radiofrequencies is desired. While ceramics and glasses are compatible with wireless RF communication, it is difficult to machine ceramics to a thin walled hermetic enclosure. The brittleness of ceramics prevents the construction of thin wall hermetic enclosures from ceramic materials.
State of the art ceramic machining can produce walls of approximately 0.5-0.7 mm thickness. For implants whose length, width, and height dimensions are typically ones of millimeters, this can represent a significant reduction in available internal volume for components such as antennas.
Hermetic enclosures known in the art, particularly those made of ceramic and/or glass materials, do not lend themselves to efficient use of limited space. Non-metal hermetic enclosures known in the art are typically manufactured via planar processing technology, such as low temperature cofired ceramic processes, laser machining, ultrasonic machining, Electronic Discharge Machining (EDM), or Micro Electro Mechanical Systems (MEMS) fabrication techniques. These techniques are capable of processing ceramics and glasses with tight control of feature resolution. However, sidewalls of an implant package made with these techniques often require use of a dicing saw or laser to separate the implant package from the remaining substrate. Due to manufacturing constraints and the need for mechanical strength, implant package sidewalls made by these methods are typically 0.3 mm-0.5 mm thick. Alternative manufacturing approaches, such as the molding or machining of ceramic, are typically limited to minimum sidewalls of 0.5-0.7 mm thick.
An example of a prior art hermetic implant package 10 is shown in FIG. 1. The implant package 10 includes thick sidewalls 12 that limit the space available for the internal components, in this case implant antenna 14. For example, an implant package of width 4 mm that has sidewalls 0.5 mm thick only has a maximum of 3 mm of width available for an implant antenna. FIG. 1 shows an antenna 14 that is placed into the implant package from an opening at the top of the package. To complete the implant package, a top layer 16 is connected or bonded to the implant package and sealed as shown in FIG. 2A. For pressure-sensing implant packages known in the art, the top layer is typically either a capacitive pressure sensor itself, a thin membrane that is directly part of a sensing electronic circuit, or a thin membrane that communicates pressure from the environment to the inside of the implant package via an incompressible liquid or gel. Manufacturing techniques known in the art are capable of routinely processing membranes to thicknesses of 0.025-0.1 mm. Many variations of the FIG. 1-2 architecture exist in the prior art, including the method of etching a cavity in half of a housing to create the thin wall on top of the coil, and then bonding the two housing halves vertically. This is depicted in the sketch of FIG. 2B, where the upper housing half 999 has a cavity etched into it to create the thin membrane.
Other prior art exemplifies wireless implant architectures of the type shown in FIG. 1 and FIG. 2, where the thin pressure sensitive membrane is in a plane that is perpendicular to the coil's axis. U.S. Pat. No. 7,574,792 (O'Brien), U.S. Pat. No. 6,939,299 (Petersen), and U.S. Pat. No. 4,026,276 (Chubbuck) all teach implantable pressure sensors with coil antennas, and hermetic housings with at least one deformable pressure-sensitive wall. In all these cases, the pressure-sensitive walls of the housings are perpendicular to the coil axis, and the walls located outside the coil perimeter are rigid, structural, and relatively thick. In these architectures, total coil area is limited by the need for a relatively thick structural wall outside the coil perimeter.
To improve implantable wireless sensors, it is desirable to have a hermetic enclosure with thin walls outside the coil antenna perimeter, thus maximizing the internal dimension that most constrains antenna size.